PURPOSE:To miniaturize and stabilize an endless belt controller by using a system in which a skew-advancement detection signal level is divided into three levels, the intermediate level is made into an insensitive zone, and a solenoid for vertically moving rollers is operated at upper and lower levels. CONSTITUTION:In a belt skew-advancement detector 24, the contact condition betwen a lever 26 and a belt 11 is changed according to the skew-advancement condition of the belt 11, the lever 26 turns around a support shaft 28, a light detector 27 moves between an opaque portion 25a, a semi-transparent portion 25b, and a transparent portion 25c, and detection signal are sent out according to each condition. Of these signals, signal for the semitransparent portion 25b is regarded as an insensitive zone by a control circuit and the control circuit does not operate, but signals positioned in the opaque portion 25a and the transparent portion 25c are judged as a skew-advancement condition. The solenoid for vertically moving conveyer rollers (not illustrate) is put into on-off action, and the roller positions are corrected. Only one skew-advancement detector is thus sufficient, and the miniaturization and stabilization of the belt position controller can be attained.